Rob French (KC4UPR)
So! An update!
I have found 3 ways I can make the hum go away. Any of these on their own appear to work: (1 and 2 are with just the Teensy USB plugged in)
(1) Unplug the laptop from it's power supply... so clearly, the laptop's AC power is a player here. Hum goes away.
(2) Short the ground side of the transformer input to the ground side of the transformer output. I.e. tie my grounds together across the transformer. Hum goes away (but, this doesn't seem like the right solution, because then why am I using the transformer, if the grounds aren't isolate?)
(3) (What I observed previously) Plug in both the Teensy USB and also the Raduino USB. So this is also a case of the two grounds being tied together.
I've got to hit the sack, but do these observations help at all?